1) i believe it's the leftmost icon on the bottom right, next to the servant list and inventory/ shop.  along with the options to summon either an imp or hobgoblin servant, there's also the option to open a submenu where you get a list of heroes that you can use gems to summon.

gems are the "currency" the altar uses.  you can get them as a reward for defeating heroes, by chance when a servant walks into the altar room, and i believe they are an objective you can send servants into the overworld for.

2)  what?  of course you can.  personally, i find that having more than one servant wandering without a job is overkill.  the only servant that matters is the one in the chokepoint from the entrance of the dungeon, the one that's going to be actually fighting.  you're probably not going to be invaded more than once in the time it takes for an overworld mission to complete.  well, unless you instigate the invasion yourself, of course.

everyone else, everyone who actually has a job, is already earning EXP (though at a fairly slow rate unless you happen to have both traits for exp gain on the servant,) and all that extra gold you'll eventually find you have is best spent on buying the exp crystal items.  this, combined with sending servants out on missions is more than EXP enough to keep most of the dungeon on pretty even footing.  with a steady rate of progress, even.